The ability to refrigerate, or cool matter, has become not only a necessity but a highly valued luxury in modern culture.
The process of refrigeration is used in a wide variety of domestic and industrial applications. Refrigeration is considered today to be the most valuable method of protecting foods from spoilage, and is used to cool air in air conditioning systems, to produce ice for cooling drinks, and to provide a means for recreation, e.g. ice skating, artificial snow, etc. Without doubt, the ability to refrigerate efficiently is of great value in today's world.
Most contemporary refrigeration systems work in the same fashion. A refrigerant vapor, usually freon, is compressed by a compressor into a liquid in the condenser. The liquid refrigerant is then passed through a flow control device to an evaporator or cooling coil, in which a reduction in the pressure accompanied by vaporization occurs. Refrigeration results from the absorption of heat during vaporization in the cooling coils. The vaporized refrigerant is then drawn back into the compressor and the cycle is repeated.
If the refrigeration system is being used to cool air within a closed environment such as a freezer, cooling cabinet, or cool room, the air is circulated over the evaporator cooling coils by a fan. As the air passes over the evaporator coils, the heat in the air is absorbed by the coils by virtue of the refrigerant inside, thus rendering the air cold.
During the vaporization phase, ice forms on the surface of the evaporator coils due to inevitable moisture present in the air. Once ice forms on the coils, less heat is available to the surface of the coils, for absorption by the refrigerant.
The less heat available for absorption on the surface of the coils, the less efficiently the vaporization process works, and hence, the system.
Removal of the ice which forms as a natural result of the refrigeration process, is critical to the efficient operation of such a system.
Most refrigeration systems solve the problem of ice formation on the cooling coils by incorporating a heating means in close proximity to the coils. The heating means is operated for a few minutes, periodically, and melts the formed ice. Since most of the defrosting devices used until now were non-adaptive, i.e. do not base initiation and termination of the heating means on actual environmental information associated with the presence or absence of ice, more problems arose.
Most current, non-frost refrigeration systems use a time-based defrosting mechanism, known as a defrost timer. This mechanism consists of a small timing device which energizes the heating means, in a cyclic fashion, based on fixed time intervals. Refrigeration manufacturers, usually define this time interval to be most optimal for their particular unit under worst-case conditions. As a result, since most common refrigerators are not exposed to worst-case conditions, the time interval set in the defrost timer is unnecessarily too short. By having too short a time interval, the defrost cycle is instigated often, and wastes energy.
Today, when energy efficiency has become important, and better methods of regulating energy consumption have become critical, such defrost control systems, employing timers are becoming obsolete.
As a result, several solutions have been realized to meet energy efficiency requirements.
One example of a defrost control system is disclosed in German patent DE2721521. This device is provided with a main air conveying channel and an auxiliary bypass channel.
There is a heat register in front of the refrigeration system for heating a cross-section of the air flow at an even rate. When the cooling coils are free of ice, the air passes smoothly through the coils. Since the heat register maintains a constant temperature, the amount of energy required to maintain that temperature is measurable. In the event of icing on the cooling coils, the free passage of air over and through the coils is interrupted. Since the heat register is not being cooled by the flow of the air through the coils, which is obstructed by the icing of the coils, the register requires less energy to maintain its fixed temperature. The reduction in the amount of energy required to heat the register up to its set temperature indicates a build up of ice on the cooling coils, and the defrosting cycle is initiated.
The disadvantage of this system is associated with its low sensitivity. This system does not initiate the defrosting action directly upon detection of ice, per se', but rather "waits" until the flow of air over and through entire area, occupied by the cooling coils, is sufficiently impeded due to the appearance of ice.
In another example of a defrosting system, described in U.S. Pat. No. 4,191,026, the defrost cycle is initiated by sensing the pressure drop across a heat exchanger. This method is implemented by installing a small diameter pipe within a wall of the duct, downstream of the heat exchanger so that during normal operation, an air current is drawn through the pipe. The pipe's interior is constantly heated so that the temperature of the heated current remains constant as long as the air volume flowing through the pipe remains constant.
As the pressure drop across the heat exchanger increases, as a result of frost buildup, relatively more air is drawn through the pipe and the temperature of the heated air in the pipe decreases, resulting in the generation of an appropriate signal to initiate the defrost cycle.
As can be readily appreciated, the disadvantages of this system are similar to those described above, in that it is not capable of detecting ice itself and initiate the defrosting action thereupon.
Besides low sensitivity inevitably associated with the monitoring of such effects, like pressure drops, flow rate variations, etc. induced by frost formation which covers the entire surface of an evaporator coil, the known systems are not pre-selective in the sense that they do not allow initiation of a defrosting cycle based on the appearance of frost in a particular location of the coil.
Further, the above defrost control systems require the establishment of a steady-state environment within the refrigerator to work efficiently. They might be unreliable in domestic applications where the aerodynamic characteristics within the cooling chamber's environment change often, e.g. due to the opening of a freezer door.
It should be pointed out that the known defrost control systems usually consist of a large number of components. These systems are bulky, require additional space within the refrigerator, and can not be retrofitted without significant alterations to existing refrigeration units.